Process control systems, like those used in chemical, petroleum or other processes, typically include one or more process controllers communicatively coupled to one or more field devices (e.g., process control devices) via analog, digital or combined analog/digital buses. The field devices, which may be, for example, instruments, valve positioners, switches and transmitters (e.g., temperature, pressure and flow rate sensors), perform process control functions within the process such as opening or closing valves and measuring process control parameters. The process controllers receive signals indicative of process measurements made by the field devices and then process this information to generate control signals to implement control routines, to make other process control decisions, and to initiate process control system alarms.
Information from the field devices and/or the controller is usually made available over a data highway or communication network to one or more other devices or systems, such as operator work stations, personal computers, data historians, report generators, centralized databases, etc. Such devices or systems are typically located in control rooms and/or other locations remotely situated relative to the harsher plant environment. These devices or systems, for example, run applications that enable an operator to perform any of a variety of functions with respect to the process implemented by a process control system, such as viewing the current state of a process, changing an operating state, changing settings of a process control routine, modifying the operation of the process controllers and/or the field devices, viewing alarms generated by field devices and/or process controllers, simulating the operation of the process for the purpose of training personnel and/or evaluating the process, etc. Often, access to the devices or systems is also based on whether they are powered (e.g., not capable of communications without loop power).